ANAEROBIC CO-DIGESTION OF MICROALGAE WITH FOOD WASTE AND WASTEWATER SLUDGE

Abstract

This research sought to optimize anaerobic co-digestion of microalgae biomass harvested from a wastewater treatment pond facility with locally-available wastes. The goal was to produce high methane yields and stable digestion without the need for supplemental alkalinity addition. A key research question was if algae digestion could be improved via the synergistic effects of co-digestion. Cell disruption to increase digestibility was not pursued due to its relatively high mechanical complexity and high energy use. For the wastewater treatment ponds studied, the most practical co-substrates identified were municipal wastewater sludge and food waste (sorted organic municipal waste). Although wastewater sludge does not have a particularly high carbon:nitrogen (C:N) ratio, it readily and stably digests and is available in large quantities at wastewater treatment plants. This research investigated the methane productivity of algae co-digestion with municipal wastewater sludge and food waste in semi-continuous bench-scale anaerobic digesters at 37.5˚C. Digesters fed pure algae biomass loaded at a rate of 4 g Volatile Solids (VS)/L-day with a 20-day residence time exhibited stable digestion and yielded an average of 0.23 L CH4/g VS Introduced. For digesters that contained algae biomass in the feed, the greatest methane yield of 0.40 mL CH4/g VSin was observed in a digester containing 50% algae co-digested with both sorted organic municipal waste (40%), and municipal wastewater sludge (10%) at a loading rate of 2 g VS/L-day with a 20-day residence time. While adding co-substrates increased yields in all digesters, prevention of ammonia toxicity did not appear to be the mechanism. Instead, the co-substrates simply increased the concentration of readily-digestible organic carbon, leading to increased methane yields and productivities. For algae biomass, total ammonia nitrogen concentrations of 3370 mg/L did not appear to inhibit methane yield. Digesters with the same feed contents and residence time loaded at 2 and 4 g VS/L-d had similar yields but total ammonia nitrogen concentrations of 1740 and 3370 mg/L respectively. From the data from these laboratory studies, descriptive models were developed for ammonia nitrogen, alkalinity, volatile fatty acids, yield, biogas quality, and volatile solids destruction. The variables from the descriptive models with p-values above 0.05 were then used to create a compact model.